1. Field of the Invention
The present invention relates to electroacoustic transducers, such as piezoelectric receivers, piezoelectric sounders, piezoelectric speakers, and piezoelectric buzzers, and also relates to a method for retaining piezoelectric diaphragms.
2. Description of the Related Art
In apparatuses such as portable telephones, electroacoustic transducers have been widely used as piezoelectric receivers. Generally, an electroacoustic transducer of this type includes an unimorphic diaphragm having a circular metal plate and a circular piezoelectric ceramic plate provided with electrodes, one of the electrodes being bonded to the metal plate. The diaphragm is retained at its periphery in a circular casing in which the peripheral area is enclosed by a cover. Such an electroacoustic transducer is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-107593 or Japanese Unexamined Patent Application Publication No. 7-203590.
A circular diaphragm applied to the known electroacoustic transducer is restrained around the entire periphery thereof, whereby a maximum deflecting point P is disposed only at a central point of the diaphragm, thereby reducing the displacement. A problem with the known electroacoustic transducer is that the sound pressure that is produced by the energy generated from the displacement is small relative to the energy input for the deflection.
To overcome the problems described above, preferred embodiments of the present invention provides an electroacoustic transducer that generates high sound-pressure.
A preferred embodiment of the present invention provides an electroacoustic transducer including a diaphragm having a substantially rectangular metal plate, and a substantially rectangular piezoelectric plate having electrodes on an upper surface and a lower surface thereof, at least one of the electrodes being bonded to the substantially rectangular metal plate, a support member including retaining parts for retaining two shorter sides of the diaphragm, an adhesive having a Young""s modulus after curing between about 4.0xc3x97104 N/m2 and about 5.0xc3x97106 N/m2 and arranged to connect the two shorter sides of the diaphragm to the retaining parts, an elastic sealant arranged to seal gaps between the two longer sides of the diaphragm and the support member, and wherein the diaphragm is arranged to vibrate in a longitudinal bending mode when a predetermined electrical signal is applied between the metal plate and the electrode provided on the upper surface of the piezoelectric plate.
According to the above described structure and arrangement of preferred embodiments of the present invention, two shorter sides of the substantially rectangular diaphragm are fixed to retaining parts of the support member, and the gaps between two longer sides of the substantially rectangular diaphragm and the support medium are sealed by the elastic sealant. The diaphragm is deflected in a longitudinal bending mode by inputting a predetermined electrical signal between the metal plate and its opposing electrode provided on a surface of the piezoelectric plate. That is, the diaphragm vertically vibrates with two longitudinal ends being supporting points which are fixed to the support medium. The two longer sides of the substantially rectangular diaphragm, which are elastically sealed by the elastic sealant, do not limit the diaphragm deflection.
The displacement caused by deflection of a circular diaphragm is small because the diaphragm is fixed to a support medium at its periphery, whereby the maximum deflecting point P is disposed only at a central point, as shown in FIG. 1A. On the other hand, the displacement caused by deflection of a substantially rectangular diaphragm is large compared with that of a circular diaphragm, because maximum deflecting points P are disposed, as shown in FIG. 1B, along an intermediate line between both longitudinal ends of the diaphragm, which enables a higher sound pressure. In other words, a substantially rectangular diaphragm is more easily miniaturized than is a circular diaphragm, when obtaining the same sound pressure level.
An epoxy-based adhesive generally used for affixing has a Young""s modulus after curing on the order of about 107 to about 106 N/m2. When both longitudinal ends of the diaphragm are fixed to a support member by such a hard adhesive, the displacement by deflection of the diaphragm cannot be large because both longitudinal ends of the diaphragm are excessively restrained. When applying a soft adhesive having a Young""s modulus after curing lower than about 4.0xc3x97104 N/m2, the entire diaphragm can vibrate in a nearly free state. In a completely free state, the displacement cannot be large because the diaphragm vibrates with node points at approximately one-sixth of its overall length from each longitudinal end.
FIG. 2 is a graph showing the relationship of the Young""s modulus after curing of an adhesive to the displacement of a diaphragm, wherein two longer sides of the diaphragm are in a free state, and the electrical signal to be applied is a non-resonant region voltage signal.
The graph in FIG. 2 shows that the displacement is very large when the Young""s modulus after curing of an adhesive is between about 4.0xc3x97104 N/m2 and about 5.0xc3x97106 N/m2, while there is a sharp decrease in displacement when the Young""s modulus exceeds about 5.0xc3x97106 N/m2.
The adhesive for fixing two longitudinal ends of the diaphragm to the support medium, according to the present invention, has a Young""s modulus after curing of about 4.0xc3x97104 to about 5.0xc3x97106 N/m2. The diaphragm which vibrates in a longitudinal bending mode with two longitudinal ends being supporting points can provide a larger displacement when an adhesive with a Young""s modulus after curing of about 4.0xc3x97104 to about 5.0xc3x97106 N/m2 is applied than when the ends are restrained or when they are in a free state. A diaphragm thus arranged can produce high sound pressure.
FIG. 3 is a graph showing the relationship of the Young""s modulus after curing of an elastic sealant to the displacement of a diaphragm. The graph shows two cases, namely, a case in which an adhesive with a Young""s modulus after curing of about 4xc3x97105 N/m2 is applied to fix two shorter sides of the diaphragm, and the other case in which an adhesive with a Young""s modulus after curing of about 4xc3x97109 N/m2 is applied to fix the same. The electrical signal to be applied is a non-resonant region voltage signal.
The graph in FIG. 3 shows that the displacement is very large when the Young""s modulus after curing of the sealant is about 5.0xc3x97106 N/m2 or less, while it shows a sharp decrease in displacement when the Young""s modulus after curing of the sealant exceeds about 5.0xc3x97106 N/m2. The displacement does not change in the range of the Young""s modulus after curing of the sealant being below about 4xc3x97105 N/m2.
Therefore, preferably the Young""s modulus after curing of the elastic sealant for sealing the gaps between two transversal ends of the diaphragm and the support medium is no more than about 5.0xc3x97106 N/m2. That is, the elastic sealant is applied only to prevent air from passing through the diaphragm, therefore, the Young""s modulus thereof is set to be as low as possible so as to apply the least possible restraint on the deflection of the diaphragm in a longitudinal bending mode.
The Young""s modulus of an adhesive which is higher than that of the elastic sealant provides preferable characteristics when the diaphragm is placed in a bending vibration in a longitudinal bending mode.
Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.